Aging is a process in which all individuals of a species undergo a progressive decline in vitality leading to death. In metazoans, aging at the level of the whole organism is clearly evident. Whether the aging of an organism is genetically programmed, or represents the effects of entropy over time is not clear. Consistent with the possibility of a genetic program are mutations which alter the aging process. In humans the genetic diseases progeria and Werner's syndrome cause premature aging in affected individuals. In the earthworm C. elegans, a gene, age-1, has been described which directly or indirectly affects the life span of the animal (Friedman, D. B. and Johnson, T. E., Genetics 18:75-86 (1988)). A further issue open to speculation is how the aging of the entire organism relates to the aging of individual cells and cell types within the organism.
That individual cells within mammals do senesce was demonstrated in the findings of Hayflick, who showed that primary human diploid fibroblasts (HDFs) would grow in culture for about 50 population doublings, and then all the cells in the population would stop dividing (Hayflick, L. and Moorhead, P. S., Exp. Cell Res. 25:585-621 (1961); Hayflick, L., Exp. Cell Res. 37:614-636 (1965)). Cells arrest in the G1 phase of the cell cycle and contain a 2N chromosomal complement (Cristofalo, V. J., et al., Exp. Gerontol. 24:367 (1989)). This in phase, or clonal, senescence of the HDFs is accompanied by a characteristic morphological change; cells enlarge as they senesce (Angello, J. C., et al., J. Cell. Physiol. 132:125-130 (1987) and Cristofalo, V. J. and Kritchevsky, D., Med. Exp. 19:313-320 (1969)). In fact, this direct correlation between cell size and senescence can be demonstrated by incubating young HDFs in low serum-medium, in which they enlarge, but do not leave the G1 phase of the cell cycle (Angello, J. C., et al., J. Cell. Physiol. 140:288-294 (1989)). When these cells are returned to medium containing adequate serum for cell division, their program of senescence has been advanced compared to smaller cells which have divided the same number of times.
Cell fusion studies between old and young HDFs indicate that senescence is dominant. In short term hybrids, initiation of DNA synthesis in the young nucleus is inhibited after the young cell has been fused to a senescent HDF (Norwood, T. H., et al., Proc. Natl. Acad. Sci. USA 71:2231 (1974)). In fact, injection of polyA+ RNA from the senescent HDF into the young cell inhibits DNA synthesis (Lumpkin, C. K., Jr., et al., Science 232:393 (1986)), suggesting that the senescent HDF activated a gene or genes that encoded dominant inhibitory proteins. In complementation studies that involve fusing various "immortal" cell lines, four genes were identified which were involved in immortalization (Pereira-Smith, O. M. and Smith, J. R., Proc. Natl. Acad. Sci. USA 785:6042 (1988)). The dominance of senescence appears to conflict with the view that shortening of telomeres, a phenomenon observed during passage of fibroblasts (Harley, C. B., et al., Nature 345:458 (1990)), causes senescence.
In several lower eukaryotes, senescence has been demonstrated and linked to changes in mitochondria. In Podospora, cell senescence is strongly associated with the excision and amplification of segments of mitochondrial DNA (Cummings, D. J., et al., J. Mol. Biol. 185:659-680 (1985) and Koll, F. et al., Plasmid 14:106-117 (1985)). In Neurospora (Bertrand J., et al., Cell 47:829-837 (1986)) and Aspergillus (Lazarus, C. M., et al., Eur. J. Biochem 106:663-641 (1989)), senescent cells also contain rearrangements in their mitochondrial DNA. In all of the above examples, the senescent phenotype is dominant and is inherited cytoplasmically.
In the budding yeast, Saccharomyces cerevisiae, cells divide asymmetrically, giving rise to a large mother cell and a small daughter cell. By micromanipulating the daughter away from the mother at each cell division, it was shown that the mother divided a fixed number of times, and then stopped (Mortimer, R. K. and Johnston, J. R., Nature 183:1751-1752 (1959)). Life span was thus defined by the number of divisions mother cells had undergone, and not by chronological time. Further, a number of cell divisions in the life span of the mother, while fixed (varying over a Gompertz distribution (Pohley, J.-J. Mech. Ageing Dev. 38:231-243 (1987)), could differ from strain to strain (ranging from about 15 to 30) (Egilmez, N. K. and Jazwinski, S. M., J. Bacteriol. 171:37-42 (1989)). Thus, senescence in budding yeast as in HDFs is not a stochastic process, but has some underlying genetic basis.
Senescence in yeast is like senescence in HDFs in other ways as well. Like HDFs, yeast mother cells have been shown to enlarge with age (Mortimer, R. K. and Johnston, J. R., Nature 183:1751-1752 (1959) and Egilmez, N. K., et al., J. Gerontol. Biol. Sci. 45:B9-17 (1990)). In addition to their large size, aging mother cells also divide more slowly than young cells (Egilmez, N. K. and Jazwinski, S. M., J. Bacteriol. 171:37-42 (1989)). A further analogy to HDFs is that the senescent phenotype is also dominant in yeast. Mating a young yeast cell to an old one generates a diploid with a limited potential for cell division (Muller, I., J. Microbiol. Serol. 51:1-10 (1985)). In addition, daughters of old mothers display elongated cycling times for the first few divisions after separation from the old mother (Egilmez, N. K. and Jazwinski, S. M., J. Bacteriol. 171:37-42 (1989)). Evidently, the senescence substance is inherited by the daughter cell and slowly degraded or diluted in subsequent cell cycles.
The senescence of yeast mother cells thus has similarities to what occurs in primary HDFs; however, there is one important difference. In yeast at each cell division the daughter cell has regained the capacity for a full life span, whether derived from a younger or older mother cell (Muller, I., Arch. Mikrobiol. 77:20-25 (1971)). This "resetting" in daughters may be intertwined with the mechanism that generates asymmetry at cell division. In any case, "resetting" argues against one category of hypothesis for aging; namely that aging results from the accumulation of errors in protein synthesis, the error catastrophe theory (Orgel, L. E. Nature 243:441 (1973)). Because daughter cells derived from old mothers have functional mitochondria (Muller, I. and Wolf, F., Mol. Gen. Genet. 160:231-234 (1978)), this resetting also shows that senescence is not due to rearrangements in the mitochondrial genome.
By varying the growth rate of cells, it was demonstrated that the key parameter in determining the life span in yeast is number of divisions, and not chronological time (Muller, I., et al., Mech. Ageing Dev. 12:47-52 (1980)). This finding led to the idea that senescence could be due to an accumulation of bud scars in mother cells. Bud scars are deposits of chitin that stay with the mother cell after each cell division (Cabib, E., et al., Curr. Top. Cell. Regul. 8:1-32 (1974), and Pringle, J. R., et al., Meth. Cell Biol. 31:357-435 (1989)). Several lines of evidence have argued against the idea that bud scars cause aging. First, varying the surface to volume ratio of isogenic yeast strains by varying their ploidy did not affect life span (Muller, I., Arch. Mikrobiol. 77:20-25 (1971)). Second, increasing the surface area by mating an old cell to a young one did not endow the diploid with an increased potential for division (Muller, I., J. Microbiol. Serol. 51:1-10 (1985)). Third, induction of chitin synthesis and deposition in the cell wall did not decrease the life span of cells (Egilmez, N. K. and Jazwinski, S. M., J. Bacteriol. 171:37-42 (1989)). Thus, senescence in yeast has gross features similar to the aging process in mammalian cells. It is therefore reasonable to speculate that the molecular mechanisms of aging might be similar in yeast and mammalian cells, particularly in light of striking parallels in basic cellular mechanisms in yeast and mammalian cells. In the field of transcription, for example, there has emerged strong mechanistic similarities in the function of transcription factors: the yeast and mammalian TATA box binding factor TFID, are interchangeable in the basal in vitro transcription reaction (Buratowski, S., et al., Nature 334:37-42 (1988)). Further, yeast and certain mammalian transcriptional activators will function normally in the heterologous host cells (see Guarente, L., et al., Cell 52:303-305 (1988) for review). Therefore, further study of aging in yeast cells may yield information concerning genes which are involved in senescence, and ultimately may shed light on the aging process in mammalian cells.